The ion concentration polarization (ICP) phenomenon at micronanofluidic interfaces has been extensively utilized to preconcentrate low-abundance biological samples. Although preconcentration by ICP is robust, its multiphysics phenomenon does not permit a clear prediction of the preconcentration conditions and sites. Here, we present a new method for spatiotemporally defining preconcentration, which can generate target-condensed plugs in a very specific region (<100 μm) regardless of the operating conditions (time, applied voltage, ionic strength, and pH). In contrast to previous devices that use only ion depletion, this device uses merged ICP zones with opposite polarity, i.e., ion depletion and ion enrichment. In this regard, ICP is initiated between two line-patterned cation exchange membranes. When voltage is applied across two membranes, an ion depletion (enrichment) zone occurs on the anodic (cathodic) side of the membranes. Two ICP zones are then merged and confined between the membranes. Consequently, the preconcentration action is also confined between the membranes. We demonstrate that fluorescent dyes are always preconcentrated at the designated location at all lengths of operating time and at broad voltage (0.5&#8211;100 V), ionic strength (1&#8211;100 mM KCl), and pH (3.7&#8211;10.3) ranges. This device successfully condenses proteins up to 10000-fold in a specific region of the channel (100 × 50 × 10 μm3) in 10 min. This work not only characterizes the unique scientific phenomenon of ICP overlapping but also opens the possibility of integrating ICP preconcentrators into commercial analysis equipment, which requires a known, stationary preconcentration site.